The phrase 'nine eleven' immediately reminds us of the terrorist attacks of 11 September 2001. On that day, terrorists from Al-Qaeda crashed airplanes into the World Trade Center in New York City, killing nearly 3000 people.
This phrase 'nine eleven' also reflects the American habit of writing a date with the month before the day - whereas in most European countries, dates are written the other way around.
The event of 'nine eleven' had a great emotional impact on people all over the globe. It would be possible to study this impact through diary entries written at the time. For this puzzle, we have a collection of diary entries from the past 100 years (between 1-1-1920 and 1-1-2020). Unfortunately, the diary pages are jumbled up. Each diary author has his own habit for the way they write dates. A date like 12-01-05 would be interpreted in the UK as 12 Jan 2005, but in the United States as 1 Dec 2005. In some countries (e.g. Hungary, Japan) people write dates with the year first so this would be: 5 Jan 2012. Or even (not so common): 1 May 2012. But at least each person is consistent, the same person always writes a date the same way. (Note that the year can never be in the middle)
Your task is to figure out who wrote about that tragic event in their diaries.
Diary entries are grouped by written dates. Each entry is followed by a list of persons that wrote those numbers in their diary, even though they might be referring to different days. Considering the test input:
16-05-18: Margot, Frank
02-17-04: Peter, Elise
06-02-29: Peter, Margot
31-09-11: Elise, Frank
09-11-01: Peter, Frank, Elise
11-09-01: Margot, Frank
By a process of elimination, we can figure out that
This means that both Peter and Margot wrote about the events of 'nine-eleven' in their diary, but not Frank or Elise.
To arrive at your answer, sort the names alphabetically and join them with spaces. The answer to the test-input is Margot Peter
By the way: writing year, then month, then day, as is common in e.g. Hungary and Japan, has certain advantages. For example, a plain text sort of dates written this way, will order dates neatly as expected. Use it in file names and have your files always neatly organized by date. Writing dates this way is also the standard set by ISO-8601. But despite standardization efforts, date formats remain a source of confusion.
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